This invention relates generally to the sealing of reclosable plastics bags, and is particularly concerned with sealing jaws appropriate for use on a form/fill/seal machine.
In form/fill/seal machines a film of plastics material passes through heat sealing jaws which apply heat and pressure to effect sealing at appropriate locations. Many bags made on form/fill/seal machines incorporate a zipper which has interengaging male and female profile members so that the bag is reclosable. The zipper strip is applied to the film material in advance of the heat sealing jaws. A problem which is encountered is that the position of the zipper strip on the host material can vary either over the course of time or in an essentially random manner, for example due to changes in the operating system. However, it is important that the zipper strip is sealed to the host material at a particular point in order for the closure to be effective.
It is therefore an object of the present invention to provide a sealing jaw which will enable sealing to be effected at the correct position as the host material moves intermittently through the sealing jaws, regardless of slight variations in the position of the zipper strip on the host material.
In accordance with the present invention this is achieved by the provision of a heat sealing jaw comprising a body provided with a plurality of spaced heat sealing wires, and a plurality of pressure switches associated with the wires and operable when pressure is imparted thereto to effect heating of a predetermined wire or wires.
Preferably, the heat sealing wires are spaced at a distance of the order of 0.1 mm from each other. Preferably, the wires are impulse sealing wires.
Also in accordance with the invention there is provided apparatus for the sealing of a bag of plastics material, which comprises guide means to direct a film of plastics material along a sealing path, and at least one heat sealing jaw as claimed in any of claims 1 to 5 located adjacent to said path and displaceable into contact with the film.
Preferably, the apparatus comprises a pair of said heat sealing jaws located one on each side of said path and aligned so as to be engageable on opposite sides of a double film of material.
Also in accordance with the invention there is provided a method of sealing a bag in a form-fill-seal machine, which comprises positioning lengths of zipper strip at intervals along a movable film of plastics material, forming and filling the bag material, and guiding the formed and filled bag material into a sealing path on opposite sides of which is located a first pair of heat sealing jaws for effecting a top seal of the bag and a second pair of sealing jaws for effecting sealing of the film to the zipper strip, at least one of said second pair of jaws being a heat sealing jaw in accordance with any of claims 1 to 5.
Preferably, the second pair of sealing jaws effect sealing of the film to a flange or flanges of the zipper strip which extend laterally from a pair of interengageable male and female closure members.